1. Field of the Invention
The present invention relates to an organic electroluminescent device (hereinafter, abbreviated as an “organic EL device”) which can be used as a planar light source or as a display device.
2. Description of the Related Art
Attention has been made to an organic electroluminescent device in which a light-emissive layer is constituted from an organic compound, because such a device can ensure a large area display at a low driving voltage. To highly increase the efficiency of organic EL devices, Tang et al. of Eastman Kodak Company, as is disclosed in Appl. Phys. Lett., 51, 913 (1987), have successfully achieved an EL device which can exhibit a high luminance and sufficient efficiency during practical use, i.e., a luminance of 1,000 cd/m2 and an external quantum efficiency of 1% at an applied voltage of not more than 10 volts, when the EL device produced has a structure in which organic compound layers having different carrier transporting properties are laminated to thereby introduce holes and electrons with a good balance from an anode electrode layer and a cathode electrode layer, respectively, and the thickness of the organic compound layers is controlled to be not more than 2,000 Å.
In the development of such high efficiency EL devices, it has been already acknowledged that the technology for introducing electrons from a cathode electrode layer and holes from an anode electrode layer into an organic layer of the EL devices without generating an energy barrier is important. In Tang et al., described above, to reduce an energy barrier which can cause a problem when electrons are introduced from a metal electrode to an organic compound which is generally considered to be an electrically insulating material, magnesium (Mg) having a low work function (3.6 eV: 1 eV=1.60218×10−19 J) is used. The work function referred to herein is based on the data described in CRC Handbook of Chemistry and Physics, 64th Edition. However, since magnesium is liable to be oxidized and instable and also has poor adhesion to the surface of the organic material, Tang et al. have suggested to use magnesium alloyed with silver (Ag: work function of 4.6 eV), since silver is relatively stable, and thus has a high work function and good adhesion to the surface of the organic material. Magnesium and silver are co-deposited to form an alloy. Reference should be made to the Kodak patents concerning organic EL devices, because the history until Tang et al. developed the use of the magnesium alloy is described therein in detail.
Referring to Kodak patents, the initially issued Kodak patents such as U.S. Pat. Nos. 4,356,429 and 4,539,507 teach that the low work function metal useful in the formation of a cathode electrode layer of the organic EL devices includes Al, In, Ag, Sn, Pb, Mg, Mn, and the like. Namely, the low work function metal is not defined with reference to its work function values in these patents. Recently issued Kodak patents such as U.S. Pat. Nos. 4,885,211, 4,720,432 and 5,059,862 teach that the required driving voltage can be lowered with reduction of the work function of the metal used in the cathode electrode layer. Moreover, it is also disclosed that the low work function metal is defined as a metal having a work function of less than 4.0 eV and any metal having a work function greater than 4.0 eV can be used as a mixture with the low work function metal having a work function of less than 4.0 eV which is rather chemically instable, to form their alloy, thereby giving a chemical stability to the resulting alloyed cathode electrode layer.
The stabilizing metal is referred to as a higher work function second metal, and candidate examples thereof include Al, Ag, Sn and Pb which are described as the low work function metal in the initial Kodak patents cited above. The inconsistencies in the disclosures between the initial and later patents show that the Kodak patents have been invented as a result of repeated trial and error at the initial stage of development. Furthermore, in the Kodak patents described above, it is disclosed that the alkaline metals having the lowest work function, should be removed from the candidate examples of the cathode metal, even though they can exhibit excellent function in principle, because they have an excessively high reactivity for achieving the stable driving of the EL devices.
On the other hand, a group of researchers of Toppan Printing Co. (cf. 51st periodical meeting, Society of Applied Physics, Preprint 28a-PB-4, p. 1040) and a group of researchers of Pioneer Co. (cf. 54th periodical meeting, Society of Applied Physics, Preprint 29p-ZC-15, p. 1127) have discovered that if lithium (Li; work function: 2.9 eV), which is an alkaline metal and has a lower work function than that of Mg, and was excluded from the claims of the Kodak patents, is used and is alloyed with aluminum (Al: work function: 4.2 eV) to form a stabilized electron injection cathode electrode layer, a lower driving voltage and a higher emissive luminance in comparison with those of the EL device using the Mg—Ag alloy can be obtained in the EL devices. Furthermore, as is reported in IEEE Trans. Electron Devices, 40, 1342 (1993), the inventors of the present invention have found that a two-layered cathode electrode layer produced by depositing lithium (Li) alone at a very small thickness of about 10 Å on an organic compound layer, followed by laminating silver (Ag) onto the deposited Li layer is effective to accomplish a low driving voltage in EL devices.
In addition, recently, the inventors of the present invention have successfully found, as is reported in Appl. Phys. Lett., 73 (1998) 2866, “SID97DIGEST, p. 775”, Japanese Unexamined Patent Publication (Kokai) No. 10-270171 and the US counterpart thereof, U.S. Pat. No. 6,013,384, that in EL devices, if an alkaline metal such as lithium, an alkaline earth metal such as strontium or a rare earth metal such as samarium are doped into an organic layer adjacent to the cathode electrode layer in place of doping the same into the metal of the cathode electrode layer, a driving voltage can be reduced. This is considered to be because an organic molecule in the organic layer adjacent to the electrode is changed to the corresponding radical anion as the function of metal doping, thus largely reducing a barrier level to the electron injection from the cathode electrode layer. In this case, even if a higher work function metal having a work function greater than 4.0 eV such as aluminum is used as the metal of the cathode electrode layer, it becomes possible to reduce a driving voltage in EL devices. In addition, it has been confirmed as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-332567 that higher work function electrode materials such as ITO, which are conventionally used in the formation of the anode electrode layer and are considered to be the most undesirable for the formation of the cathode electrode layer, can be used as a cathode material to provide a drivable light-emissive device.
Moreover, the inventors of the present invention have proposed organic EL devices in Japanese Unexamined Patent Publication (Kokai) Nos. 11-233262 and 2000-182774. These EL devices are characterized in that an organic layer in a portion adjacent to the cathode electrode layer is formed from an organometallic complex compound containing at least one metal ion of an alkaline metal ion, an alkaline earth metal ion and a rare earth metal ion or is formed from a mixed layer of the organometallic complex compound and an electron-transporting organic compound and the cathode electrode layer is formed from the electrode material which includes a thermally reducible metal capable of reducing an alkaline metal ion, an alkaline earth metal ion and a rare earth metal ion, contained in the organometallic complex compound in the mixed layer, in vacuum, to the corresponding metal (cf. The 10th International Workshop on Inorganic and Organic Electroluminescence, p. 61; Jpn. J. Appl., phys., Vol. 38(1999) L1348, Part 2, No. 11B, 15 November, Reference 12; Jpn. J. Appl., Phys., Vol. 41(2002) pp. L800).
In the electron, injection layer having the above structure, during vapor deposition of the thermally reducible metals such as aluminum and zirconium under a vacuum, the thermally reducible metals can be vaporized in atomized state, i.e., in highly reactive conditions, and be deposited onto the organometallic complex compound, thereby reducing metal ions in the complex compound to the corresponding metal state and liberating the reduced metals therein. Furthermore, the reduced and liberated metals can cause an in-situ doping and reduction of the electron-transporting organic compound existing near the reduced and liberated metals (the reduction caused herein means the reduction defined by Lewis and thus acceptance of electrons). Accordingly, as in the above-described direct metal doping process, the electron-transporting organic compound can be changed to radical anions. Namely, according to this method, aluminum is selected, not by its level of the work function as in the conventional methods, but by the thermally reducible ability under vacuum conditions. Furthermore, a similar phenomenon has been observed and reported with regard to inorganic compounds containing a low work function metal ion such as alkaline metal ions (cf. Appl. Phys. Lett., Vol. 70, p. 152 (1997); and IEEE Trans. Electron Devices, Vol. 44, No. 8, p. 1245 (1997)).
As can be appreciated from the above-described historical descriptions of the electron injection technologies, in the development of organic EL devices, there have been continuous attempts to improve the electron injection electrodes and improve the method of forming an electron injection layer in an interface with the cathode electrode layer. As a result, the emission efficiency of the EL devices could be drastically improved and also it became possible to drive the EL devices at a low voltage. Accordingly, at present, the electron injection has been recognized to be important technologies for improving the EL device properties in the production of the organic EL devices.
Moreover, for the injection of holes into the organic layer, an indium-tin-oxide (ITO) is widely used as a transparent oxide electrode material having relatively higher work function in the formation of an anode electrode layer in the organic EL devices. ITO has been already widely used in the production of the liquid crystal display devices, and under this circumstance, it can be said that suitability of transparent electrode like ITO for EL devices is considered to be a result of the unexpected luck, because ITO is a material which is relatively appropriate for the hole injection into the organic layer because of its higher work function and also, without saying, light has to be extracted plane-wise in the EL devices. In addition to that, ITO is widely available now because LCD industry uses ITO coated glass substrate in its mass production scale.
Furthermore, Tang et al. of Eastman Kodak Company have further improved compatibility of organic layer with an anode electrode layer by inserting a layer of copper phthalocyanine (hereinafter, CuPc) having a thickness of not more than 200 Å between the anode electrode layer and the hole-transporting organic compound, thereby enabling the operation of the EL devices at a low voltage and at a more stable state (cf. Kodak patents, cited above). Furthermore, a group of researchers of Pioneer Co., Ltd., have obtained similar effects by using star-burst type arylamine compounds proposed by Shirota et al., of Osaka University (cf. Appl. Phys. Lett., 65, 807 (1994)). Both of the CuPc and the star-burst arylamine compounds have the characteristic of having an ionization potential (Ip) smaller than that of ITO and their hole mobility is relatively large, and thus they can improve stability of the EL device during continuous driving, as a function of improved interfacial compatibility, in addition to low-voltage driven property.
In addition, a group of the researchers of Toyota CRDL, Inc., have proposed an organic EL device in which a metal oxide such as vanadium oxide (VOx), ruthenium oxide (RuOx) or molybdenum oxide (MoOx), which have a larger work function than ITO, is deposited at a thickness of 50 to 300 Å by sputtering on an ITO layer to thereby reduce an energy barrier generated during hole injection from the ITO layer (anode electrode layer) to the organic layer (cf. Japanese Patent No. 2824411). In this EL device, the driving voltage can be considerably reduced in comparison with the sole use of ITO.
Similarly, the assignee of the present invention, as is disclosed in Japanese Patent Application Laid-open Nos. 10-49771 (Japanese Patent Application Laid-open No. 11-251067 (corresponding U.S. Pat. No. 6,423,429B2) and 2001-244079 (corresponding U.S. Pat. No. 6,589,673 B1), has succeeded with regard to hole injection from the anode electrode layer in improving the hole injection property of a EL device if a sort of a Lewis acid compound and an organic hole-transporting compound are properly selected and are mixed in a appropriate ratio using a co-deposition method to form a hole injection layer (cf. Jpn. J. Appl. Phys., Vol. 41(2002) L358).
In this EL device, since a Lewis acid compound capable of acting as an oxidation agent for the organic compound is being doped into a layer of the organic compound adjacent to the anode electrode layer, the organic compound is retained as molecules in the oxidized state and as a result, an energy barrier during hole injection can be reduced, thereby ensuring to further reduce a driving voltage of the EL devices in comparison to the prior art EL devices. Moreover, if a suitable combination of the organic compound and the Lewis acid compound is selected in this chemical doping layer, an increase of the driving voltage can be avoided, even if a thickness of this layer is increased to an order of micrometers, in contrast to the prior art layer constituted from only undoped organic compounds, and thus a dependency of the driving voltage upon the layer thickness of the chemical doping layer can be removed in the EL devices (cf. Preprint of 47th periodical meeting of Japanese Society of Polymer, Vol. 47, No. 9, p. 1940 (1998)). In addition, as is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-244079 (Japanese Patent Application No. 2000-54176), the above-described Lewis acid-doping layer may be used to adjust an optical path length of the EL device to thereby enable the layer to act as a controlling layer of an emission spectrum profile which can be utilized to improve a color purity of the display image.
Regarding the above-described hole injection technologies, their features and drawbacks will be summarized as follows. First, the hole injection layer having mixed therein a Lewis acid compound, suggested by the assignee of the present invention, has characteristics which could not be observed in other hole injection layers such as the characteristic that a driving voltage of the EL devices is not substantially increased along with the increase of the thickness of the hole injection layer because of the low resistivity of the hole injection layer, and the layer is considered to be the most effective hole injection layer among the available hole injection layers. On the other hand, generally, many of the Lewis acid compounds are chemically instable and therefore they suffer from poor storage stability. Further, the inventors of the present invention have found that the Lewis acid compounds may slightly deteriorate the current efficiency (or quantum efficiency) of the EL devices. Similarly, the inventors of the present invention have found that the hole injection layer cannot act as a buffer layer for reducing a process damage during formation of the electrode layers. The hole injection layer using an organic compound having a small ionization potential, suggested by Tang et al. and Shirota et al., can improve a compatibility with the anode electrode layer, however, due to the upper limit of the applicable layer thickness, a layer design (including layer thickness) of the EL devices cannot be unlimitedly changed.
Similarly, the lamination of a metal oxide having a large work function on the anode electrode layer, suggested by Toyota CRDL Inc., suffers from limitation in the applicable layer thickness due to low light transmittance of the metal oxide, and the limitation that substantially all of the exemplified compounds can only be deposited with a sputtering method.
In any case, hitherto, the hole injection layer of the present invention has not yet been suggested which is characterized by having no layer thickness dependency of the driving voltage because of a low resistivity of the hole injection layer, enabling a maintenance of the high current efficiency (quantum efficiency) and having a property or function as a process damage-diminishing layer during formation of the electrode layers, in addition to the function as the hole injection layer.